1. Field of the Invention
The invention is in the field of dispensing apparatus.
2. Description of the Prior Art
Typical prior art pumps presently available, especially for use in airless spraying, are shown in U.S. Pat. No. 3,680,981 to Wagner and U.S. Pat. No. 3,788,554 to Guise. Several patents disclose an interaction between the driven fluid pressure and the pumping fluid section of a diaphragm pump. The consideration in these systems has been to use the driven fluid pressure to control a valve for returning hydraulic fluid to the sump. The purpose for this is to reduce the loading and wear on the pump. Such systems are shown in U.S. Pat. No. 2,971,465 to Cailland, No. 2,753,805 to Boivinet, No. 3,433,161 to Vetter, No. 3,416,453 to Feuillebois, and No. 3,411,673 to Mann. Mann also shows a system wherein the driven fluid inlet check valve is opened in response to high driven fluid output pressure rather than affecting the hydraulic side of the pump system. U.S. Pat. No. 1,651,964 to Nelson shows a compressed air output controlling the hydraulic fluid return in a diaphragm pump. In U.S. Pat. No. 3,775,030 to Wanner, fluid output pressure controls a valve which returns fluid to the inlet of the pump.
None of the above patents apprehend the use of hydraulic section pressure to control or unload the driven fluid side of the system. In the past many injuries have occurred while the driven fluid section of an airless spraying unit was pressurized, even after the power to the motor was disconnected, from an operator attempting to perform maintenance on the unit, and therefore there has been a need for means for automatically depressurizing the driven fluid section of the system.
A fluid dispensing gun having some pressure-responsive valve stem movement is shown in U.S. Pat. No. 3,410,491 to Malec. Malec, however, does not disclose a safety gun mechanism responsive to pressure as set forth herein.